Skip to main content

Full text of "Handbook Of Chemical Engineering - I"

See other formats


MECHANICAL SEPARATION
307
grammatically the three modes of employing centrifugal force in centrifugal machines.
Reparations Suitable to Centrifugal Force.—The use of centrifugal machines, owing to their cost and relatively low capacity, should be limited as far as possible to finishing operations. They are suited to drying operations where heat would destroy or injure the material to be dried or where in would be dangerous as with nitro-cellulose. In a very general way the greater the bulk of the individual unit of the solids introduced into a centrifugal machine the better it is suited to centrifugal separation. Also in a general way the less the percentage of liquid absorbed by the material to be separated the more suited it is to centrifugal separation. Where the liquid is not absorbed by the solid as oil on metal chips, shavings and small machined castings, the centrifugal separation offers a ready and expeditious and often the only means of removing the liquid. Where the liquid is not absorbed by or held by the solids as mother liquor in separating the crystals from it comparatively large proportions of liquid are permissible in centrifugal separation. One of the most extensive uses to which centrifugal machines may be put is in getting rid of pore moisture and chemicals in liquid form in textiles, leather, nitro-cellulose, etc., following washing, dyeing or vat treatment of some sort. When other separation means would be out of the question or too prolonged as in separating sticky viscid liquids from solids such as molasses from muscovado the centrifugal separator effects a removal which is rapid and complete. Finely divided amorphous material cannot be separated in a centrifugal machine used as an extractor or filter. In such cases where the conditions are proper for it the bowl can be substituted for the basket, the solids adhering to the inner surface of the former being removed after they form coats of the desired thickness.
The principal advantage of the centrifugal machine lies when it can be used as an extractor as the solids can be completely freed of liquid in one operation.
DETAILS OF CENTRIFUGAL EXTRACTOR.   FIG. 29
Size of basket .................. Revolutions per minute ........	30 in. 1,150	36 in. 950	42 in. 800	48 in. 700	54 in. 600	60 in. 500
Velocity in feet per minute ...... Centrifugal force of 1 Ib. at periphery, pounds ...............	9,040 563	8,954 461	8,796 3S2	8,796 334	8,483 276	7,854 213
Approximate shipping weight. . . I Forsopowcr of motors Type "E . "	2,200 3	2,700 3	4,000 5	5,000 7>2	6,200 7H	7,600 10
Details of Centrifugal Separators and Use.—No continuously operating separator of the extractor type has been successful. The stumbling block to attaining this end has been the continuous removal of the solids formed but a choice of machine best suited to the work will often increase capacity. If there be not much liquid to be extracted the batch to be placed in the machine may be worked in a removable basket.
Hydro extractors with top drives are the most common kind. The drive may be by motor, direct-connected steam engine or by belting. Figure 29 shows a motor-driven machine. Balancing of this machine is done through three swivel bolts connected to the stand. It is necessary to have the center of gravity of the basket and its contents as nearlv invariable in position as possible otherwise destructive stresses